[jayec]

Have a look at job sites to see what a salaried position with your experience would get you. Use that to work out the hourly rate you would be paid and add 25-30% to cover your expenses for the time you spend on admin tasks or tasks that don't bring in any money. Multiply your new hourly rate by how long you think it will take you to complete a task. Over time you'll get better at estimating your time.

If you need to purchase something for the project I generally sell it to the client at cost and work the cost of the purchase into the overall price. Most clients would expect you to add a markup to purchases but they're pleasantly surprised when you don't. You still get your money and you get a happier client in the end. Everybody wins.